The answer to your question;-

More to the point, why have they unchallenged powers of censorship here?

is because it is their forum. Paid for, configured, and administered by them. All you have to do is set up a similar forum, and you will have unchallenged powers of censorship yourself. Maybe you could set up a forum for the people, and elect the moderators, or put every forum management decision to the vote. That's your choice. The moderators here will never please everyone, so all they can do is do what they think is right.